1. Field of the Invention
The present invention relates, in general, to spacer grids used for receiving and supporting fuel rods in nuclear fuel assemblies and, more particularly, to a side-slotted nozzle type double sheet spacer grid used in such fuel assemblies and designed to effectively deflect and mix coolants together so as to improve the heat transfer effect between the fuel rods and the coolants, and also designed to enhance its fuel rod support performance so as to effectively protect the fuel rods from vibration and fretting wear, and enhance its strength to effectively resist laterally directed forces acting thereon, and remarkably improve the spring performance of its support parts directly supporting the fuel rods, thus accomplishing desired soundness of the fuel assemblies.
2. Description of the Prior Art
In typical nuclear reactors, a plurality of elongated nuclear fuel rods 125 are parallelly arranged at regular intervals in a fuel assembly 101 having a square cross-section. In such a case, for example, fourteen, fifteen, sixteen or seventeen fuel rods 125 are arranged along each side of the square cross-section at regular intervals, thus forming a 14xc3x9714, 15xc3x9715, 16xc3x9716, or 17xc3x9717 array as shown in FIG. 1.
In order to receive and support the fuel rods 125 within the nuclear fuel assembly 101, a plurality of single sheet spacer grids 110 are used. In order to produce each single sheet spacer grid 110, a plurality of inner strips intersect together at right angles to form an egg-crate pattern, prior to being welded at their intersections. Thereafter, the periphery of the spacer grid 110 is encircled with four perimeter strips. The top and bottom of the fuel assembly 101 are, thereafter, covered with top and bottom pallets 111 and 112. Therefore, the nuclear fuel assembly 101 is protected from any external loads acting on the top and bottom thereof. In the fuel assembly 101, the spacer grids 110 and the pallets 111 and 112 are integrated into a single structure using a plurality of guide tubes 113. The elongated fuel rods 125, received and supported within the fuel assembly 101 by the spacer grids 110, are typically fabricated such that a fissionable fuel material, such as uranium core 114, is contained in a hermetically sealed elongated zircaloy tube, known as the cladding.
The above spacer grids 110 are each fabricated as follows. As best seen in FIG. 2, two sets of single sheet inner strips 115 and 116, each having a plurality of notches at regularly spaced positions, are assembled with each other by intersecting the two sets of strips 115 and 116 at the notches, thus forming a plurality of four-walled cells. Each of the cells has four intersections 117. The assembled strips 115 and 116 are, thereafter, welded together at the intersections 117 prior to being encircled with the perimeter strips 118. A desired spacer grid 110 with such four-walled cells is thus fabricated.
As shown in FIG. 3, a plurality of positioning springs 119 and a plurality of positioning dimples 120 are integrally formed on or attached to the inner strips 115 and 116. In such a case, the springs 119 and the dimples 120 extend inwardly with respect to each of the four-walled cells. The dimples 120 are more rigid than the springs 119. In each of the four-walled cells, the positioning springs 119 force a fuel rod 125 against associated dimples 120, thus elastically positioning and supporting the fuel rod 125 at four points within each of the cells.
In such a typical nuclear fuel assembly 101, a plurality of single sheet spacer grids 110 having the above-mentioned structure are regularly arranged along the axes of the fuel rods 125, as shown in FIG. 1, thus receiving and supporting the fuel rods 125 within the fuel assembly 101 at multiple points. That is, the spacer grids 110 act as a multi-point support means for receiving and supporting the fuel rods 125 within a nuclear fuel assembly 101.
In the typical nuclear fuel assembly 101, the positioning springs 119 elastically and slightly force the fuel rods 119 against the dimples 120 such that the fuel rods 125 are slidable on the support points of both the springs 119 and the dimples 120 when the fuel rods 125 are elongated due to thermal expansion or irradiation-induced growth of the fuel rods 125 within the fuel assembly 101.
When the fuel rods 125 are fixed to the spacer grids 110 at the support points of the grids 110, the fuel rods 125 may be bent at portions between the support points of the grids 110 due to the thermal expansion or the irradiation-induced growth, so that the fuel rods 125 may fail to maintain the designed intervals between them. In such a case, the cross-sectional areas of coolant passages, defined between the fuel rods 125 to allow coolant to flow through them as shown in FIG. 4, are partly increased or reduced.
In some typical nuclear reactors using water as coolant, such as in the case of the nuclear reactors currently used in Korea, water receives thermal energy from the fuel rods 125 prior to converting the thermal energy into desired electric energy through a plurality of processes.
During an operation of a nuclear fuel assembly 101 of such a reactor, water or liquid coolant is primarily introduced into the fuel assembly 101 through an opening formed on the core supporting lower plate of the reactor. In the fuel assembly 101, the coolant flows upwardly through the coolant passages, defined between the fuel rods 125, and receives thermal energy from the hot fuel rods 125.
The sectioned configuration of the coolant passages formed in the fuel assembly 101 is shown in FIG. 4.
In a conventional nuclear reactor, the amounts of thermal energy generated from different portions of a nuclear fuel assembly 101 are not equal to each other. Since the fuel assembly 101 has a rectangular cross-section, with a plurality of elongated, parallel fuel rods 125 having a circular cross-section closely set within the assembly 101 while being spaced apart from each other at irregular radial intervals, the temperature of coolant flowing around the fuel rods 125 is variable in accordance with positions of coolant currents relative to the rods 125.
That is, the amount of thermal energy received by water flowing around the corners 123 of each four-walled cell is less than that received by water flowing around the fuel rods 125. The coolant passages of typical fuel assemblies 101 thus undesirably have low temperature regions.
Such low temperature regions reduce the thermal efficiency of the nuclear reactor. The coolant passages of the fuel assemblies 101 may also have partially overheated regions at positions adjacent to the fuel rods 125 having a high temperature. Such partially overheated regions deteriorate soundness of the fuel assemblies 101.
In order to prevent such partially overheated regions from existing in a nuclear fuel assembly, it is necessary to design the spacer grid such that a uniform temperature distribution is generated in the fuel assembly. The spacer grid must also be designed to effectively deflect and mix the coolant within the fuel assembly. Such effectively mixed coolant ensures a uniform increase in enthalpy, and maximizes the core output.
Typical examples of such designed spacer grids are disclosed in U.S. Pat. No. 4,728,489 (corresponding to Korean Patent Publication No. 91-1978) and U.S. Pat. No. 4,692,302 (corresponding to Korean Patent Publication No. 91-7921).
In the spacer grids disclosed in the above-mentioned patents, so-called xe2x80x9cmixing bladesxe2x80x9d or xe2x80x9cvanesxe2x80x9d are attached along the upper edges of the intersecting strips of each spacer grid, and are used for mixing coolants within the fuel assembly. That is, the mixing blades or vanes allow the coolant to flow laterally, in addition to the normal flow in an axial direction, as shown in FIG. 3, and so the coolants are effectively mixed with each other between the coolant passages and between the lower temperature regions and the partially overheated regions of the fuel assembly.
In the prior art, the techniques for mixing the coolants with each other between the coolant passages and between the lower temperature regions and the partially overheated regions of the fuel assembly using such mixing blades or vanes are classified into two types: the first technique using large-scaled mixing blades for creating a lateral flow of coolant and the second technique using vanes provided at the intersections for creating a swirl flow of coolant. In the first technique, the coolants, axially flowing along the elongated fuel rods within a fuel assembly, partially collide against the large-scaled mixing blades so as to flow laterally, in addition to the normal flow in the axial direction. In the second technique, several vanes are provided at the intersections of the spacer grid for creating the swirl flow of coolants.
However, the conventional two techniques for mixing the coolants with each other between the coolant passages using such mixing blades or vanes are problematic in that the pressure of the coolants is undesirably reduced in inverse proportion to the expected coolant mixing effect, and so the two techniques are undesirably limited in their coolant mixing effects. That is, a wake stream, disturbing the main flow of the coolants, or a vortex flow of the coolants, generated at positions around the bent portions of the mixing blades or vanes, is enhanced in proportion to the size or the bending angle of the mixing blades or vanes, which is enlarged for enhancing the lateral flow or swirl flow of the coolants within a nuclear fuel assembly. Therefore, the pressure of the coolants in such a case is reduced to limit the enhancement of the desired lateral flow or the desired swirl flow of the coolants. This limits the size and bending angles of the mixing blades or the vanes, and limits the thermal hydraulic efficiency of the nuclear fuel assembly.
In addition, a nozzle type double sheet spacer grid, comprising two sets of intersecting inner strips each consisting of two sheets specifically deformed and integrated together to define coolant channels (nozzles) between the two sheets, has been proposed. In the nozzle type double sheet spacer grid, the inlet and outlet of each nozzle are inclined relative to the axes of the fuel rods at a predetermined angle of inclination, thus producing a swirl flow of coolant at the inlet and outlet of the nozzle. Such a swirl flow of coolant improves the heat transfer effect between the fuel rods and the coolant within a nuclear fuel assembly.
Such a nozzle type double sheet spacer grid having the two sets of double sheet inner strips is referred to U.S. Pat. No. 4,726,926, and U.S. Pat. No. 6,130,927 (corresponding to Korean Patent No. 265,027).
The above nozzle type double sheet spacer grids, designed to form a lateral flow of coolants or to deflect and mix the coolants within a nuclear fuel assembly, are somewhat advantageous in that they more effectively mix the coolants and improve the heat transfer effect between the fuel rods and the coolants within a nuclear fuel assembly. However, such a conventional nozzle type double sheet spacer grid is problematic in that the lateral flow or mixing of coolants regrettably vibrates the elongated, parallel, closely spaced fuel rods within the fuel assembly.
In the conventional spacer grids for nuclear fuel assemblies with the fuel rods 125 supported by both the positioning springs 119 and the positioning dimples 120 within the four-walled cells of the grids 110, the fuel rods 125 during an operation of a nuclear fuel assembly 101 briefly and periodically interfere with the intersecting strips of the spacer grids due to vibrations caused by the lateral flow of coolants. When the fuel rods 125 are so vibrated over a lengthy period of time, the claddings of the fuel rods 125 are repeatedly and frictionally abraded at their contact parts at which the fuel rods 125 are brought into contact with the springs and dimples of the spacer grids. The claddings are thus reduced in their thickness so as to be finally perforated at said contact parts. Such an abrasion of the fuel rods is so-called fretting wear in the art.
Detailed description of such a fretting wear may be referred to U.S. Pat. No. 4,702,881 (corresponding to Korean Patent Publication No. 94-3799).
The laterally directed force caused by the mixing blades of the spacer grids is in proportion to the coolant mixing effect, and directly affects the heat transfer effect of nuclear reactors. However, such a laterally directed force of the mixing blades also proportionally increases the amplitude of vibration of the fuel rods. This may cause damage to the fuel rods.
While designing the spacer grids having such mixing blades, the fuel rod support parts, such as positioning springs and dimples, are recognized as very important factors in recent years.
Other important factors to consider while designing the spacer grids for use in nuclear fuel assemblies are both the fuel rod supporting function of the spacer grids and the buckling strength capable of resisting such a laterally directed force acting on the grids.
During an operation of a nuclear reactor, the fuel assemblies may be vibrated laterally due to load acting on the fuel assemblies and this causes interference between the fuel assemblies. Therefore, the spacer grids of the fuel assemblies may be impacted due to such interference between the fuel assemblies as disclosed in U.S. Pat. No. 4,058,436.
In the prior art, the spacer grid""s buckling strength, resisting a lateral load acting on the spacer grid, is undesirably reduced since the strips of the spacer grid must be partially removed, for example, through a stamping process at a plurality of portions so as too form positioning springs and dimples. Such cut-away portions reduce the effective cross-sectional area of the spacer grid capable of resisting impact, and reduce the buckling strength of the spacer grid.
In a spacer grid disclosed in U.S. Pat. No. 5,243,634, the positioning springs are individually integrated with an associated grid strip at one point, thus forming a cantilever structure. Such a cantilever spring is more flexible than an equal-arm spring, which is integrated with a grid strip at both ends thereof to form an equal-arm structure.
In the spacer grid disclosed in the above-mentioned U.S. Pat. No. 4,726,926, the deformed portions, provided on the sheets of the intersecting grid strips for forming the coolant channels (nozzles), collaterally act as positioning springs used for receiving and supporting the fuel rods within the four-walled cells of the grid.
That is, the sheets of each grid strip are not cut away, but deformed to form such coolant channels (nozzles), so that the nozzle type positioning springs are integrated with a grid strip at both sides and both ends thereof. The strength of the nozzle type springs is thus higher than that of conventional equal-arm springs. The nozzle type springs are thus greatly reduced in their elastic ranges, so that the nozzle type springs act as dimples rather than springs. The nozzle type springs are thus so-called xe2x80x9cnozzle type dimplesxe2x80x9d.
Therefore, the above-mentioned spacer grid, having such nozzle type dimples and supporting the elongated fuel rods using only the nozzle type dimples without having springs, is problematic in that the dimples may cause the fuel rods to be undesirably bent when the fuel rods are elongated due to the irradiation-induced growth during an operation of the reactor. In addition, this spacer grid is reduced in its elastic range, wherein the spacer grid effectively and elastically supports the fuel rods in the fuel assembly, and so the spacer grid may be apt to lose its spring function during a grid manufacturing process, during fuel rods installing process, or when the fuel rods are elongated due to the irradiation-induced growth during an operation of the nuclear reactor. In such a case, the spacer grid may undesirably lose its function of effectively receiving and supporting the fuel rods within a nuclear fuel assembly and restricting undesired vibration of the fuel rods.
In an effort to overcome the above-mentioned problems experienced in the conventional single sheet and double sheet spacer grids, the inventors of the present invention proposed a nozzle type double sheet spacer grid for nuclear fuel assemblies in U.S. patent application Ser. No. 09/862,383 (corresponding to Korean Patent Application No. 2001-14474).
The above nozzle type double sheet spacer grid has mixing blades used in a conventional single sheet spacer grid, in addition to having coolant channels (nozzles) used for mixing the low temperature coolant with the high temperature coolant in the same manner as disclosed in U.S. Pat. No. 6,130,927 (corresponding to Korean Patent No. 265,027), thus having advantages expected from both the mixing blades of the conventional single sheet spacer grid and the coolant channels of the conventional nozzle type double sheet spacer grid, and improving thermal efficiency of the fuel assemblies. The above nozzle type double sheet spacer grid also has an enhanced spring function of supporting the fuel rods within a fuel assembly, thus reducing fretting wear of the fuel rods caused by flow-induced vibration of the fuel rods within the nuclear fuel assembly.
In addition, the above nozzle type double sheet spacer grid for nuclear fuel assemblies is designed such that the intersecting strips are not cut away at any portion, thus maintaining a desired effective sectional area thereof, and thereby having a desired buckling strength capable of effectively resisting lateral load acting thereon. The nozzle type double sheet spacer grid thus improves the thermal hydraulic strength and mechanical strength of the fuel assembly. The above nozzle type double sheet spacer grid also increases the elastic range of the positioning springs, and increases the number of fuel rod contact points and fuel rod contact areas, thus effectively and smoothly receiving and supporting the fuel rods within the fuel assembly, different from the other conventional double sheet spacer grids. The above nozzle type double sheet spacer grid supports the fuel rods at the increased number of contact points, and so the spacer grid effectively supports an elastic displacement of the fuel rods, different from conventional spacer grids supporting such elastic displacement by only the positioning springs, thus overcoming the problems experienced in the conventional nozzle type positioning springs having excessively high strength.
However, the above nozzle type double sheet spacer grid is problematic in terms of the performance of nozzle type springs directly supporting the fuel rods in a fuel assembly. Regardless of various efforts to improve the spring performance, the nozzle type springs of the above double sheet spacer grid inevitably have high strength in comparison with the springs of conventional single sheet spacer grids, so that the double sheet spacer grid may fail to allow the fuel rods to smoothly slide on the support points of the springs in the case of irradiation-induced growth of the fuel rods, thus undesirably resulting in a bending of the fuel rods. In addition, the double sheet spacer grid regrettably has a reduced elastic range, wherein the grid effectively supports the fuel rods in the fuel assembly, and so the double sheet spacer grid may lose its spring function during a grid manufacturing process, during fuel rods installing process, or when the fuel rods are elongated due to the irradiation-induced growth of the fuel rods during an operation of a nuclear reactor. In such a case, the double sheet spacer grid undesirably loses its function of effectively restricting undesired vibration of the fuel rods, so that the fuel rods may be severely damaged due to flow-induced vibrations caused by the flow of coolants in the fuel assembly.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies, which is designed to remarkably enhance the performance of springs supporting the fuel rods in a fuel assembly, in addition to providing the function of coolant channels (nozzles) used for mixing the low temperature coolant with the high temperature coolant, thus effectively protecting the fuel rods from fretting wear caused by flow-induced vibrations within the fuel assembly.
Another object of the present invention is to provide a side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies, which is designed to overcome the problems of conventional nozzle type springs having high strength, and have the advantages of conventional spacer grids having springs and dimples capable of effectively supporting the elastic displacement of the fuel rods different from spacer grids having only springs, thus allowing increased elastic displacement of the fuel rods in a nuclear fuel assembly.
In order to accomplish the above objects, the present invention provides a side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies, comprising a plurality of inner strips intersecting each other at a predetermined angle to form a plurality of four-walled cells to receive and support a plurality of fuel rods in the spacer grid, each of the inner strips comprising a plurality of unit strip parts each of which is fabricated by integrating two unit sheet parts together into a single structure, such that the two unit sheet parts face each other and a nozzle type coolant channel with an inlet and an outlet is defined between the two unit sheet parts, wherein each of the unit sheet parts is provided with a slot longitudinally formed on each side surface of a spring which is projected from the unit sheet part to support a fuel rod within a four-walled cell.
In the above side-slotted nozzle type double sheet spacer grid, the slot has a width in the range from 0.35 mm to 0.8 mm, and a length in the range from 12 mm to 16 mm.